1. Field of the Invention
This invention relates to an image data conversion method which is capable of primarily compressing at a high speed the image bit trains and to a character code/character pattern conversion apparatus which converts character codes to character patterns utilizing said image data conversion method.
2. Description of the Prior Art
In image processing apparatus, for example, facsimile equipment, the number of picture elements of a line of the draft paper of size A4 (corresponding to one scanning line of a CRT, not a line of a sentence) is determined as 1728 bits or 2432 bits for the size A3. In the case of facsimile equipment, the amount of data to be transmitted is reduced since a photo-electric image conversion output is not transmitted directly but in general a band compression is carried out. Also, in the case of transmitting code output from computer to the facsimile equipment, band compression is required in the same way in place of the character pattern itself. If an access is made to a character generator (CG) with the code signal transmitted from the computer and the character pattern signal such as 5.times.7 or 16.times.16 dots/character is output and is subjected to the band compression for transmission to the facsimile equipment and characters are all fixed to a small constant size since the dot interval is small and constant as explained above. However, in some cases it is actually desirable to recognize that the title needs to be written in large size characters to be read easily and that the comments are written unattractively by small size characters. Therefore, it is desired to carry out character expansion and reduction processing for the output of CG.
In the case of ordinary facsimile equipment, the bit information for the image data of the pictures is converted to an intermediate code, for example, into a run length code in order to compress the bit information to be transmitted. This run length code indicates the same code bit a plurality of times and includes information indicating its length. For example, the bit train "001111000" is converted in such a way that 0 has a 2-bit length, 1 has a 4-bit length and 0 has a 3-bit length.
In the existing conversion system for converting image data into the runlength code, the image bit information is converted in to serial bits by a parallel to serial convertor and the number of bits appearing during the period between a first changing point (changing from 0 to 1 or from 1 to 0) and the next changing point is measured by means of a counter. This method results in a disadvantage in that the conversion processing requires a longer time because the serial bits must be checked or counted bit by bit and when the time required for the parallel to serial converter to output one bit is considered as t, the time n xt is necessary for converting the bit information of the image bit of n-bit into the bit length information.
Therefore, in order to improve such conversion time, it is possible, for example, to employ a system where a code conversion table is prepared and the input image data is converted to the runlength code only with an access to the table. According to this conversion table, a problem arises, where the conversion time can be improved considerably because it is determined only by the retrieval time of conversion table because the table size becomes large where the image data length in units of data to be converted such as the facsimile data is comparatively long.
For this reason, a conversion system is proposed herein which respectively solves the disadvantages of the existing bit serial conversion system by means of the conversion table.